Method for reducing vessel draft

ABSTRACT

An improved method for raising a propeller up into the hull of a marine water craft for the purpose of reducing the likelihood of underwater impact and improving shallow water operation without encountering the high efficiency losses normally associated with shallow draft drive systems or water jets. It utilizes a unique tunnel which is open bottomed and extends longitudinally in the running surface of the hull. The lead in to the tunnel, which is termed the “chute” forms the entry to a deeper portion of the tunnel located further aft. A super-cavitating propeller is positioned in the aft tunnel section. The propeller is raised vertically into the tunnel such that the propeller blade tips can be above, below or flush with the running surface of the hull but the prop shaft is always above the running surface of the hull. The invention relates to the longitudinal position of the chute with respect to the propeller, the general shape of the chute and its normal method of operation.

FIELD OF THE INVENTION

This invention is directed to the field of water craft, and inparticular to a method for reducing vessel draft of tunnel mountedsurface piercing propellers.

BACKGROUND OF THE INVENTION

The desire for propulsion systems which have little or no draft is auniversal need. Water jets and surface drives are two embodiments ofmarine propulsion systems which have shallow draft as one of theirdefining characteristics.

Jets are essentially water pumps that have been fitted into water craft.Water is brought into the an inlet which is generally composed of anupswept bend which lifts the water and another bend changes itsdirection and brings it to the face of the impeller. The impellerincreases the pressure in the water column which is needed to expel thewater through a nozzle. This event produces thrust to propel the craft.

However, water jets have always been relatively inefficient whencompared to open propellers and efforts to improve their efficiency overthe last 30 years have met with limited success. Top speed is still wellbelow the normal attainment of simple open propellers.

Air ingestion is also a problem when rough seas are encountered. Jetsgenerate pressure by slightly compressing the water they intake. Thismakes them sensitive to relatively small amounts of air in the incomingstream. Pratt & Whitney testing showed that when air volumes in theinlet flow are just a few percent of the total inlet flow, pumpefficiency drops off dramatically. In normal use in heavy seas airingestion is difficult if not impossible to eliminate from the inlet andso jets tend to unload when air is ingested and lose thrust. To combatthis problem the inlets so jets tend to be kept as short as possible butoften this length deficiency hurts performance due to excessiveseparation or turbulence in the inlet stream capture volume.

Surface drives are another way to reduce draft by bringing the propellerup vertically and operating it with only partial submergence. Whilesubmergence varies, 50% of the propeller diameter is a good average. Infixed surface drives steering is accomplished through the use of arudder behind the propeller just like an inboard. In moveable shaftsystems the propeller can be steered and trimmed for improved high speedoperation.

Unlike jets, surface drives have excellent efficiency. Efficiencies ashigh as 85% have been documented for inclined shaft, 50% submergedsuper-cavitating propellers. Inclined super-cavitating propellers arethe most efficient form of marine propulsion known.

What is lacking is a product which takes the speed advantages andefficiency of surface drives and joins them with the shallow draft andprotection from underwater impact damage afforded by water jets.

A significant amount of prior art exists which relates to raising thepropeller of a marine vessel vertically up into the hull for the purposeof avoiding or minimizing impact with an underwater obstruction.

The surface drive, invented by Hickman in his U.S. Pat. No. 1,044,176dated Nov. 12, 1912 sought to raise the propeller of a conventionalinboard up vertically to as to reduce appendage drag and improve topspeed. Many ideas followed with different approaches to utilizing thebasic advantages of surface drives. Many of these concepts extend thepropeller out the stem of the boat in either a fixed shaft mechanism ora moveable shaft mechanism but some designers sought to bring thesurfacing propeller forward of the transom and into the hull into whatis generally termed the tunnel.

Once this evolution became accepted designers sought ways to furtherreduce the amount of propeller which extends below the bottom of thehull without paying a performance penalty. Unfortunately, a performancepenalty is always paid if the propeller requires incoming water to belifted up vertically in order to operate.

Generally when a surfacing or super-cavitating propeller is brought upinto a tunnel, the tunnel is at least partially filled with air. Whenair is introduced into the hull channel ahead of the propeller there areoften methods for controlling the source, point of introduction andamount of air.

Examples of tunnel systems which are air in a tunnel with asuper-cavitating propeller can be found in prior art such as U.S. Pat.Nos. 1,534,725; 3,604,385; 3,745,963; 3,793,978; 4,015,556; 4,027,613;4,371,350; 4,406,635; 4,655,157; 4,689,026; 4,941,423; 4,977,845,

U.S. Pat. No. 1,534,725 by Macmillan shows a set of counter-rotatingpropellers (van wheels) which operate in a cavity which is filled withpressurized air to maintain the level of water in the cavity desired.Macmillan does not raise the propeller blades above the propeller whichis approximately 40-50% submerged. Macmillan also does not propose ahull channel to feed the propeller water.

U.S. Pat. No. 3,604,385, High Speed Water Craft by Mickleover concerns ahigh speed water craft which utilizes a tunnel with a propeller disposedlongitudinally inside the tunnel. Mickleover states that the forwardportion of the tunnel rises clear of the water level when the craft ison plane to admit air into the tunnel and relieve the suction fromunderneath the hull. While there is a secondary source of air forrelieving the suction (the engines exhaust) it is clearly understoodthat the primary source of air to ventilate the tunnel is comingdirectly into the front of the tunnel cavity when the forward portion ofthe tunnel rises clear of the natural water surface.

The current invention seeks just the opposite of Mickleover bypreventing air from entering the hull channel from the front of theboat. Unlike Mickleover it is the intent of this patent to have theabrupt wall located in the tunnel to be positioned below the surface ofthe water under normal operation and to achieve this we position thestep in the tunnel further aft, closer to the propeller and lower,vertically so that under normal conditions the step in the hull channelis under water due to the normal submergence of the hull when on plane.This is important to understand. The Mickleover approach fails tocontrol the submergence of the propeller in rough water conditions.Waves traveling down the forward tunnel section will cause unevenloading of the propeller.

U.S. Pat. No. 3,745,963 by Fischer has propellers in cavities in thehull with the tunnel aft of the top half of the propellers and thepropeller shafts extending below the running surface of the hull.

U.S. Pat. No. 3,793,980, by Sherman, details a tunnel mountedsuper-cavitating propeller which is positioned with approximately halfof the propellers effective diameter below the running surface of thehull. At slow speed the tunnel is filled with water which is pulled upinto the tunnel through a series of slots located in front of thepropeller. U.S. Pat. No. 4,027,613 by Wollard shows a stepped hull witha surfacing propeller aft of the step. No hull channel exists forfeeding the surfacing propeller.

U.S. Pat. No. 4,057,027 by Foster shows a super-cavitating propeller ina pocket near the stem with a short water supply duct.

U.S. Pat. No. 4,406,635 by Wuhrer seeks to improve the construction ofthe mechanism which is used to move the flow control plate in the Kruppapatent.

U.S. Pat. No. 4,655,157 by Sapp shows a stepped hull which pivots at thetrailing edge of the running surface located some distance fore ofmidship. There is no hull channel to feed water to the super-cavitatingpropeller.

U.S. Pat. No. 4,689,026 by Small shows a super-cavitating propeller in atunnel with air. There is no hull channel feeding the propeller water.The position of the propeller is approximately 50% submerged.

U.S. Pat. No. 4,977,845 by Rundquist builds on the concept by Kruppa andWurher to control propeller submergence and boat handling with movableflaps.

SUMMARY OF THE INVENTION

The present invention is related to a marine propulsion system which isdisposed in a tunnel that runs longitudinally in the bottom of a marinewater craft. The invention provides a method for raising a propeller upinto the hull of a marine water craft for the purpose of reducing thelikelihood of underwater impact and improving shallow water operationwithout encountering the high efficiency losses normally associated withshallow draft drive systems or water jets. It utilizes a unique tunnelwhich is open bottomed and extends longitudinally in the running surfaceof the hull. The lead in to the tunnel, which is termed the “chute”forms the entry to a deeper portion of the tunnel located further aft. Asuper-cavitating propeller is positioned in the aft tunnel section. Thepropeller is raised vertically into the tunnel such that the propellerblade tips can be above, below or flush with the running surface of thehull but the prop shaft is always above the running surface of the hull.The invention relates to the longitudinal position of the chute withrespect to the propeller, the general shape of the chute and its normalmethod of operation.

One object of the invention is to supply a stream of water, ofsufficient quantity and suitable cross sectional shape to efficientlyoperate with a super-cavitating propeller.

An object of this invention is that under normal operation the water issupplied in the correct position to the propeller without raising thewater.

It is another object of this invention to efficiently channel theincoming water up, vertically, to engage the super-cavitating propellerand to keep this channeled water free of air under normal operation.

Another object of this invention is to accomplish all of these improvedattributes without complex movable surfaces in the tunnel or chute.

Another object of the invention is that the propeller is higher,vertically, in the tunnel than what would be considered normal operationwhere normal operation for a surfacing propeller is defined herein ashaving approximately 50% of the diameter of the propeller below theprimary running surface of the hull.

An alternative method consists of a shroud around the propeller forprotection. A further method incorporates a guard that protects thesystem from damage.

Other objectives and advantages of this invention will become apparentfrom the following description taken in conjunction with theaccompanying drawings wherein are set forth, by way of illustration andexample, certain embodiments of this invention. The drawings constitutea part of this specification and include exemplary embodiments of thepresent invention and illustrate various objects and features thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a pictorial side view of a vessel having a tunnel mountedsurface piercing propeller with chute, shroud and guard.

FIG. 2 is a stem view of FIG. 1;

FIG. 3 is an isometric bottom view of FIG. 1;

FIG. 4 is a side view cut away to show chute, shroud and guard; sectionshowing chute: detail showing micro-step;

FIG. 5 is same as FIG. 4 showing a gear case with propeller;

FIG. 6 is aft portion of hull without shroud and guard;

FIG. 7 is aft view of FIG. 6;

FIG. 8 is a side cutaway of a twin application;

FIG. 9 is a aft view of FIG. 8;

FIG. 10 is an isometric bottom view of FIG. 8;

FIG. 11 is side view of a small craft showing curved inlet chute;

FIG. 12 is side view of a small craft showing chute parallel to keel;

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In a water craft (1) having a bow (2) and a stem (3) the general shapeof which are relatively unimportant, with a tunnel (4) which runslongitudinally fore and aft in the running surface (5) of the hull.While the shape of the tunnel shown is approximately square theinvention related to other shapes as well. Within the tunnel ispositioned a super-cavitating propeller (6) which can be secured with astrut (7) and shaft (8) arrangement or any other suitable arrangementsuch as a gear-case (15) in FIG. 5, which can secure the propeller inthe desired position in the tunnel. The tunnel extends from the stem ofthe vessel (3) forward toward the bow for a certain distance. measuredfrom forward most face of the propeller. For this distance the tunnel isrelatively unchanged in its basic cross section which is octagonal,rectangular or square but could be semi-circular as well. After thedefined distance the tunnel has an abrupt change in cross section withthe inclusion of a vertical of nearly vertical wall (9). This wallshould have a relatively sharp trailing edge radius (10) for the purposeof breaking the incoming water stream free from attachment. As long asthe water breaks free the shape of this wall can be sloped aft (16) toprovide improved reverse performance. The inlet chute (11) begins atthis point (10) and is disposed from this point forward towards the bow(1).

The chute is defined in cross section as a channel which opens downwardand is generally rectangular with a width (12) which is less than 2times the propeller diameter and a depth (13) which is less than onepropeller diameter. While shown as roughly rectangular other shapes willperform as well.

In longitudinal section the chute may employ different shapes. The inletconfiguration has differing forms according to the application. Theinlet of the chute in a craft similar to FIG. 4 could have a fairedtransition or a microstep (22) as in the detail of FIG. 4. Thismicrostep allows the water to break free and reattach as it passes thisdiscontinuity in the hull. For hulls of very high speed, light weightand relatively short overall length as shown in FIG. 11, it will bedifficult to keep the chute exit (10) under water when the craft isrunning at top speed because as vessels go faster the wetted keel or theamount of the hull which is actually in the water continues to decrease.In this case, the chute will of necessity be curved (20) FIG. 11, toutilize the Coanda effect to lift the incoming stream of water as shownin FIG. 11. In FIG. 12, when the combination of speed, weight and lengthare such that adequate free surface submergence exists such that theroof of the chute (11) is impacting incoming water under normal runningconditions then the curved inlet chute would be abandoned for one thatis straight with the preferred embodiment being a chute roof which isparallel to the keel. In this manner the inlet chute becomes a liftingsurface because the free stream water which is at rest relative to thevessel simply enters the chute and impacts its upper surface, creatinglift. It then flows down the cute and into the tunnel cavity. Unlikecurved inlets on jets which are significant sources of drag thisstraight inlet chute minimizes pressure and form drag and does not haveto lift any water.

It is to be understood that while I have illustrated and describedcertain forms of my invention, it is not to be limited to the specificforms or arrangement of parts herein described and shown. It will beapparent to those skilled in the art that various changes may be madewithout departing from the scope of the invention and the invention isnot to be considered limited to what is shown in the drawings anddescribed in the specification.

What is claimed is:
 1. A method for reducing the draft of a water vesseland supplying water flow to a surfacing propeller comprising: A)providing a vessel with a hull having a bow, stem, keel and a surfacingpropeller turned by a propeller shaft, said vessel having a bottomrunning surface defining the draft; B) providing a downwardly openingtunnel in said keel extending from said stem toward said bow, saidtunnel having a roof and depending side walls, said tunnel having achute section toward said bow, said chute section being of lesser crosssection than said tunnel, said chute and said tunnel separated by anabrupt change in cross section; C) reducing the draft of said vessel byplacing said surfacing propeller in said tunnel, said propeller shaftlocated above said running surface; D) turning said propeller shaft toprovide forward headway to said vessel; and E) providing a stream ofwater through said chute, said stream having the same cross section assaid chute whereby said stream is free of air and of sufficient quantityto efficiently operate said surfacing propeller.
 2. The method of claim1 wherein said roof is oriented parallel to said keel.
 3. The method ofclaim 1 wherein said vessel has a running angle when underway and saidroof is sloped at an angle approximate to said running angle.
 4. Themethod of claim 1 wherein said tunnel includes a protective plateextending between said side wall underneath said propeller.
 5. Themethod of claim 4 wherein a circular duct is formed in said tunnelencircling said propeller.
 6. The method of claim 1 including the stepof providing a micro-step at the chute inlet, said micro-steptemporarily separating water flow from said hull, said water flowreattaching to said hull toward said stem.
 7. The method of claim 1including the step of providing a curved chute inlet, said water flowbeing lifted above said running surface by said inlet.